A fundamental acquisition procedure in a cellular wireless system is the cell search, which is performed by a mobile terminal for obtaining time synchronization and frequency synchronization to a cell in the network and detecting its cell identity. Cell search is enabled by the detection of synchronization signals transmitted from a base station.
The cell search is regarded as a procedure demanding much complexity and power in the mobile terminal, since finding synchronization requires correlators (i.e., matched filters) performing complex valued multiplications (due to matching the received signal to a replica signal). It is therefore crucial to design the synchronization signal such that low-complex receiver implementations of the cell searcher can be used.
A further objective of the synchronization signal is to enable detection at very low Signal-to-Interference-plus-Noise-Ratios (SINRs). Low SINRs often occur at the cell edge and the coverage of the cell is implicitly dependent on whether the synchronization signal can be detected. However, low SINRs may not only occur at distances far from the transmitter. High interference situations may be common in heterogeneous network deployments, i.e., where small low-power cells (e.g. pico cells, femto cells, Home eNodeBs etc.) are deployed at the same carrier frequency as a high-power macro cell and in its coverage region.
In heterogeneous network deployments the experienced SINRs at a mobile terminal may become much smaller than what is currently seen in homogeneous macro cell deployments. It is therefore a problem to provide synchronization signals that can be detected under severe interference conditions with a dominant interferer, while at the same time exhibit a structure that allows low-complex detectors.
For such deployments, system performance can be improved by allowing a mobile terminal (e.g. a User Equipment, UE) to connect to the pico cell, although the received power from the pico cell is smaller than for the macro cell. In that case, a UE that is connected to a pico cell may experience a stronger signal (i.e., large interference) from the macro cell, which implies that the SINRs at the UE could be much less than 0 dB. This cell-association procedure is sometimes referred to as cell range expansion and can be achieved by adding a bias value in the cell selection criterion. FIG. 1 illustrates a heterogeneous network deployment, where received signal strength from a macro cell is larger than a received signal strength from a pico cell, if a mobile terminal is located in the cell range expansion region.
The macro cell is regarded as a dominant interferer for UEs that are in the range expansion zone. These severe interference situations could be handled for data channels by coordinated scheduling between the macro cell and the pico cell. However, there are typically no means for interference coordination of the synchronization channels, which makes synchronization a problem.
For example, in a 3GPP LTE Rel-10 system, a primary synchronization signal is transmitted on 62 subcarriers; 31 subcarriers directly below and above the DC subcarrier, respectively. This applies to all cells in the system. Hence, synchronization signals from different cells are always overlapping in frequency. This constitutes no major problem for homogeneous network deployments. However, for a frame synchronous heterogeneous network deployment, the system has no means for avoiding the strong interference caused by a synchronization signal from a macro cell colliding with the synchronization signal of the pico cell.
Patent documents U.S. Pat. No. 7,751,490, EP2090050 and EP1980030 disclose synchronization signals enabling low-complex receiver implementations. A centrally symmetric number sequence du[n], n=0, 1, . . . , L−1, where L is odd, has its central element n=(L−1)/2 punctured. The punctured number sequence is mapped to a set of discrete Fourier frequency coefficients Hu[l],l=0, 1, . . . , N 1, such that the Fourier coefficients are symmetric around l=0, i.e., Hu[l+p]=Hu[l−p]=Hu[l−p+N], where p is an integer and where the last equality follows from the periodicity of the discrete Fourier transform. The mapping can thus be described by
            H      u        ⁡          [      l      ]        =      {                                        0            ,                                                l            =            0                                                                                          d                u                            ⁡                              [                                  l                  +                                                            L                      -                      1                                        2                                                  ]                                      ,                                                              l              =              1                        ,            2            ,            …            ⁢                                                  ,                                          L                -                1                            2                                                                                                      d                u                            ⁡                              [                                  l                  -                  N                  +                                                            L                      -                      1                                        2                                                  ]                                      ,                                                              l              =                              N                -                                                      L                    -                    1                                    2                                                      ,            …            ⁢                                                  ,                          N              -              1                                                                        0            ,                                                elsewhere            .                              
When generating the continuous time-domain base-band signal, the discrete frequency l=0 corresponds to the DC subcarrier, i.e., the center frequency of the carrier. By such construction, the discrete time-domain base band signal
                              s          u                ⁡                  [          k          ]                    =                        1          N                ⁢                              ∑                          n              =              0                                      N              -              1                                ⁢                                                    H                u                            ⁡                              [                n                ]                                      ⁢                          W              N                              -                kn                                                          ,          k      =      0        ,    1    ,    …    ⁢                  ,          N      -      1                          W        N            =              exp        (                  -                                    j              ⁢                                                          ⁢              2              ⁢              n                        N                          )              ,          j      =                        -          1                    
becomes centrally symmetric, su[k]=su[N−k], k=1, 2, . . . , N−1. This property can be used in an implementation to reduce the number of complex valued multiplications.
Furthermore, for a complex conjugated sequence pair u and ú, where dú=du*[n], it follows that sú[k]=su*[k]. Also this property can be used in an implementation to reduce the number of complex valued multiplications.